For prevention and cure of AIDS, it is important to know the mechanism of how cell lineage for T lymphocyte is determined during embryonic hematopoiesis. BMPs specify the fate of mesodermal tissues more ventral. Hematopoietic cells are derived from the most ventral mesoderm, therefore, BMPs are hypothesized to play important roles during specification of T lymphocyte lineage in the hematopoietic tissues. Our basic strategy to prove this hypothesis is to culture embryos that have floxed alleles for BMPs or BMP receptors using whole embryo culture system, and then to microinject Cre recombinase expressing plasmids in the interested region of embryos. Through this method, we could achieve region specific gene disruption. For this purpose, first, we established the culture method for the embryos as early as day 5.5 embryos. We found that DMEM supplemented with 75% rat serum worked better than 100 % rat serum for E5.5 embryos whereas 100 % rat serum works better for E5.75 embryos. We are underway to optimize the condition to achieve the Cre-dependent gene disruption by Cre plasmids. The knowledge that will be acquired through this research may give us better understanding for the primitive and definitive hematopoiesis.